The tarrasque is another icon of D&D lore, now conveniently available in the Basic Rules. I’m on record in my article on the strongest monsters in the game with the idea that this is not the most dangerous monster. Commenters have…expressed disagreement. I stand by my assessment though. Sul Khatesh can easily defeat a tarrasque single-handedly.
That’s not saying a whole lot in some ways. As one of my players noted recently, a clay golem could pull off a similar feat, and is significantly weaker. I think that part of this is intentional design. The tarrasque was made to have weaknesses that players could exploit. A CR 30 creature that lacks flight or any ranged attacks is hugely disadvantaged in a fight against appropriately leveled PCs. That’s on top of the fact that it’s shockingly slow for a creature of its size.
The Story Can Make It Work…
If the players choose, or are forced to, confront this monster in an enclosed space they’re in trouble. If it can get its bulk into a cave, that can reduce the players options and them face it head on. Another option is to give the tarrasque other objectives. Let’s say you’ve got a tarrasque that would be more than happy to murder the party, but mostly it wants to tear down the nearby castle. It is a Siege Monster after all. They players will have to find a way to provide a more attractive target to distract the titan.
…Or You Could Give the Tarrasque Lasers
Giving a creature a ranged attack doesn’t technically effect the Challenge Rating. Give it a ranged spell attack that deals force damage with a 120 foot range. Give it the same damage roll as the claws. I’d go so far as to let it use this attack 5 times. You could also fix it’s mobility, and give it flight. It probably got that from swallowing some prior adventurer with boots of flying. My last idea is to give it the Telekinesis spell, so it can grab players and slowly drag them into range. I think I like that one best.